1. Technical Field
The present invention relates to methods and apparatuses to utilize and recover CO.sub.2 contained in combustion exhaust gas of fossil fuel such as coal in an attempt to remove carbon dioxide gas (CO.sub.2) contained in the combustion exhaust gas before it is discharged to atmosphere.
2. Background Art
Combustion exhaust gas from a combustion equipment using fossil fuel contains a large volume of CO.sub.2. In particular, exhaust gas generated from thermal power plants using coal, LNG and petroleum or exhaust gas from gas turbine generators is enormous, and CO.sub.2 contained in the exhaust gas which is simultaneously generated from the thermal power plants is also tremendous. Conventionally, a large volume of exhaust gas generated from the thermal power plants is discharged to atmosphere without recovering CO.sub.2 contained the exhaust gas. Recently, various approaches have been propsed to recover CO.sub.2. For instance, it was proposed to discharge the exhaust gas into the open sea or to convert it into useful substances for recycle.
The table below shows the volume of CO.sub.2 discharged from the thermal power plants with respect to installed capacity and generated energy. The values used in the table for installed capacity and generated energy are specified by the power construction project plan for 1995 prepared in accordance with actual results of 1985.
______________________________________ Installed Gener- ca- ated pacity energy (.times.10.sup.4 (.times.10.sup.8 CO.sub.2 discharge rate KW) KWH) Nm.sup.3 /KWH .times.10.sup.8 Nm.sup.3 Ratio ______________________________________ Coal 2300 990 0.42 416 32.1 LNG 4300 1660 0.24 398 30.8 Petroleum 5100 1600 0.30 480 37.1 Total 11700 4250 1294 100 ______________________________________
As shown in the table above, the coal-fired thermal power plant discharges 0.42 Nm.sup.3 KWH of CO.sub.2, LNG-fired thermal power plant 0.24 Nm.sup.3 /KWH and petroleum-fired thermal power plant 0.30 Nm.sup.3 /KWH, thereby evidencing the discharge of a great volume of CO.sub.2.
As discussed above, since a large amount of CO.sub.2 is expelled to atmosphere from the thermal power plants, long wave long radiation from the earth surface is absorbed by CO.sub.2 expelled. This prevents the long wave long radiation from permeating through the atmosphere, thereby warming the earth surface and lower atmosphere and causing the green-house effect and an environmental pollution problem. As the measure to prevent this green-house effect, the recovery of CO.sub.2 discharged from the thermal power plants is imperative and its effective recovery method has been investigated. However, as mentioned above, the CO.sub.2 volume discharged from the thermal power plants is enormous. In general, the concentration of SOx and NOx contained in the flue gas is just small (150 to 200 ppm at most) whereas that of CO.sub.2 is said to be some hundred thousand ppm. It is believed to be impossible to treat CO.sub.2 with the current removal technique such as the flue gas desulfurization or denitration process, and no economical technique has been developed to recover CO.sub.2 from the condition diluted with a great volume of air. As for the technique to discharge CO.sub.2 into the sea proposed as one of CO.sub.2 disposal methods, CO.sub.2 recovered in some manner must be discharged to the sea after liquefaction, which raises problems to consume extra electric power in recovering, liquefying and transporting CO.sub.2 to the offing.